feat: per-service port drift in wt new and wt setup

[pkudinov]

Summary

When wt new or wt setup allocates ports, in-use ports now drift forward by 1 instead of failing or silently skipping the slot.

• Per-service drift. Only the conflicting service moves; the slot's other services stay at their natural ports. Drift respects both OS-level binds and ports already reserved by other wt allocations in the registry.
• Listener identification. OS conflicts surface a stderr line like Port 3200 (web) in use by node[12345]; using 3201 instead. via lsof (best-effort, never throws).
• Internal conflicts (another wt slot owns the port) skip the OS probe and report reserved by slot 1 (web) instead.
• JSON mode. Drift information appears as data.portDrifts: PortDrift[] in --json payloads from both commands; stderr lines are suppressed in that mode.
• Slot selection is now purely registry-based — findAvailablePortSafeSlot (which silently skipped slots with port conflicts) is gone, so you no longer end up in a higher slot for non-obvious reasons.
• wt setup port-reuse fix. Re-running setup on an existing worktree (e.g., from the post-checkout hook) now reuses the registered ports verbatim. Previously it recomputed via the formula, which is invisible today but would silently overwrite drifted ports once drift exists.
• Cap. Drift cap is 65535 with a hard error if a service can't find a free port.

Spec: docs/superpowers/specs/2026-04-24-port-drift-allocation-design.md
Plan: docs/superpowers/plans/2026-04-25-port-drift-allocation.md

Bumps version to 0.4.1.

Test Plan

• `pnpm lint` clean
• `pnpm test` — 100 passing, 1 skipped (pre-existing docker integration)
• `pnpm exec tsc --noEmit` clean
• `pnpm build` clean
• In a downstream consumer with a real `wt.config.json`: occupy a slot's natural port (`python3 -m http.server 3100`), run `wt new test/drift-smoke --no-install`, confirm stderr reports the drift line and the worktree is created at the next port. Then `wt remove`.
• Same downstream repo: re-run `wt setup` on an existing worktree and confirm registered ports are unchanged.

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